BRASILIA (Reuters) - Brazil's Senate met on Tuesday to vote on whether to indict President Dilma Rousseff on charges of breaking budget laws, in the long-awaited start of the trial phase of impeachment proceedings against her.

With Brazil's attention focused on the Olympic Games in Rio de Janeiro, senators in the capital Brasilia began to decide the country's political future in a raucous session presided over by Chief Justice Ricardo Lewandowski that could run into Wednesday.

The impeachment of Rousseff, a former leftist guerrilla and the first woman to lead Brazil, has paralyzed Brazilian politics since the start of the year, deepening a crisis set off by a massive kickbacks and bribery scandal at state-led oil company Petrobras.

The outcome of the vote is a foregone conclusion because opponents of Rousseff, who was suspended in May, need only a simple majority in the 81-seat Senate to put her on trial.

A final verdict expected at the end of the month would require two-thirds of the votes. Media surveys of the Senate point to defeat for Rousseff and the end of 13 years of her Workers Party rule.

"We know we won't win tonight. We are focusing efforts on the final judgment by trying to win over undecided senators," Workers Party Senator Regina Sousa told reporters.

Conviction on charges of manipulating government accounts and spending without congressional approval would definitively remove her from office and confirm interim President Michel Temer, who would serve out the rest of her term through 2018.

Temer's aides are confident that 60 senators will vote to proceed with a trial, which is six more than needed to eventually convict her.

Uncertainty over Brazil's political future has hampered efforts by Temer to plug a fiscal crisis inherited from Rousseff who is blamed for driving the economy into what could be its worst recession since the 1930s.

Temer, Rousseff's conservative former vice president who took over when the Senate suspended her on May 12, has asked senators to wrap up the trial quickly so he can advance with reforms to cap public spending and rework an overly generous pension system.

Investor expectations that Rousseff would be ousted and replaced by the more business-friendly Temer have strengthened Brazil's currency and driven up shares on the Sao Paulo stock market by over 30 percent since January, placing them among the world's best performing assets.

Rousseff has denied any wrongdoing and denounced her impeachment as a right-wing conspiracy to illegally remove a government that improved the lot of Brazil's poorer classes, by using an accounting technicality as a legal pretext.

Rousseff's impeachment has stirred criticism outside Brazil, including from former U.S. presidential candidate Senator Bernie Sanders, who said she was being put on a political trial not a legal one. "To many Brazilians and observers the controversial impeachment process more closely resembles a coup d’état," Sanders said in a statement issued on Monday.

Her critics say Rousseff's interventionist economic policies and inability to govern led to the debacle in Latin America's largest country. They argue that, whatever the legal reasons for impeaching her, she should not be allowed to return to office for the good of the country.

Her supporters argued that Rousseff is being ousted by politicians who are in many cases being investigated for corruption and do not have a moral leg to stand on.

Corruption allegations forced the resignation of three of his cabinet members and could implicate Temer himself. In plea bargain testimony published by local media over the weekend, jailed construction magnate Marcelo Odebrecht reportedly claimed Temer had received illegal campaign funding.

One senator unconvinced about the legitimacy of impeaching Rousseff said he would vote for her trial because her return would be worse in a nation with a discredited political class.

"The people want new elections," Senator Cristovam Buarque said in a speech to the Senate. "The impeachment will not placate their anger."